Cysteine catabolism is dependent upon two unique enzymes that are the only known mammalian thiol dioxygenases-enzymes adding molecular oxygen to a sulfhydryl group to form a sulfinic acid. These two unique enzymes are cysteine dioxygenase (CDO), encoded by CDO1, and cysteamine dioxygenase (ADO, 2-aminoethanethiol dioxygenase), which we recently showed to be encoded by human gene C10orf22. The clinical literature and, more recently, the study of CDO polymorphisms in disease and control populations, have shown a strong association of impaired metabolism of cysteine to sulfate and taurine and/or CDO loss-of-function mutations with a variety of autoimmune and neurodegenerative diseases. CDO is one of the most highly regulated metabolic enzymes responding to diet that is known, and this robust regulation of CDO activity suggests that cysteine homeostasis is very important to the living organism. Our long-term goals are integrate molecular and organismal studies (a) to further elucidate the structure-function relations in CDO and ADO to provide insights into thiol chemistry and regulation of these enzymes and (b) to further elucidate the roles of CDO and ADO in intermediary metabolism and regulation of physiological function in healthy individuals as well as the possible roles and contributions of CDO deficiency to autoimmune and/or neurodegenerative diseases. Our specific aims related are (1) to define the catalytic mechanism of CDO through structural and spectroscopic studies of complexes of wild type and mutant enzymes in complex with substrates, products and substrate analogs; (2) to crystallize and solve the structure of wild type ADO, and carry out studies to characterize its catalytic mechanism; (3) to characterize the phenotype of CDO-knockout or CDO-deficient mice, including those with tissue- specific CDO gene disruption; (4) to determine whether adverse effects of CDO gene disruption are affected by dietary manipulations (e.g., reduced by restricted cysteine, supplemental taurine, or supplemental sulfate, or amplified by diets containing excess sulfur amino acids, low taurine, or low sulfate); (5) to assess the functional contribution of CDO expressed in specific cell types or tissues to cysteine metabolism and regulation of cysteine levels; (6) to determine whether specific mechanisms that might contribute to the development or progression or severity of autoimmune and/or neurodegenerative disease -- including reduced taurine mediated antioxidation, reduced expression of the complement regulatory protein DAF, reduced capacity for sulfation of glycosaminoglycans, and increased production of H2S -- are altered in the CDO knockout mouse model; and (7) to assess the role of ADO in cysteamine metabolism and in the biosynthesis of hypotaurine/taurine by generating and studying an ADO knockout mouse model. PUBLIC HEALTH RELEVANCE: Cysteine homeostasis is very important to the living organism, and cysteine dioxygenase, the major enzyme involved in regulating body cysteine levels, can undergo ~300-fold changes in activity in response to changes in dietary protein content with these changes being accomplished within hours of the diet change. The clinical literature and the study of polymorphisms of the gene encoding cysteine dioxygenase (CDO1) in disease and control populations have shown a strong association of impaired metabolism of cysteine to sulfate and taurine and/or CDO1 loss-of-function mutations with a variety of autoimmune and neurodegenerative diseases. Further exploration of the structure and function of cysteine dioxygenase and use of a mouse knockout model to study the effects on loss-of-function mutations of CDO1 and their modification by diet will further our ability for prediction and early diagnosis of related disease states and for prevention of disease or alleviation of disease prevention by dietary modifications.